This invention relates to position finding for vehicles such as aircraft, particularly for collision avoidance, using the standard Air Traffic Control Radio Beacon System (ATCRBS) signals to determine the positions of transponder-equipped stations within the service area of a secondary surveillance radar (SSR).
Many collision avoidance systems using the ATCRBS signals have been devised or proposed. Some simply provide an indication or alarm upon proximity of Own and Other stations; some require active signal transmissions for determination of range; others require uplink data transmissions from ground-based equipment. All are subject, to a greater or lesser extent, to production of false alarms, or missed alarms or radio signal interference, such failures occurring frequently under congested airspace conditions where least tolerable. Bearings from Own to Other stations, required for effective maneuvering to evade collision threats, have heretofore been difficult to obtain; proposed airborne directional antenna systems for this purpose have proven to be too unreliable and costly to be practical.
While North pulse transmissions from SSRs can be used to determine bearings, this invention avoids the need for so-called North pulse kits to be installed. More recently, a system meeting the needs of collision avoidance under most prevalent conditions has been successfully operated, as disclosed in our U.S. patent application Ser. No. 787,977 filed Oct. 16, 1985. However, that system is primarily useful only within the overlapping service areas of two or more SSRs. Such areas usually exist where there is enough air traffic to create an urgent need for collision avoidance systems. However, in remote or undeveloped regions where only one SSR may be active, there remains a need for an improved collision avoidance system.